This invention relates to monitoring a pellicle to determine when it wears out. More particularly it relates to methods and structures to monitor pellicles subject to exposure wavelengths that can cause pellicle thinning.
Typically people look at the pellicle in monochromatic light and look for signs that the mask pattern is “printing” onto the pellicle surface. The technique is not quantitative and not very sensitive. It also is sensitive to the wrong thing, namely thin-film interference within the pellicle, not externally measured change in optical path length. Thin film interference is sensitive to changes in nt (where n is the refractive index and t is the thickness), while external optical path length is sensitive to changes in (n−1)t.
An example of another system that is intended to determine pellicle life is provided by Japanese Application Number JP19870111206 19870506 entitled “Exposure Device” by Mitsubishi Electric Corporation. In that published application a light flux from an optical system is focused on a photo detector for measuring light intensity at wafer level. The light intensity is measured with and without a pellicle to determine the transmissivity of the pellicle. When the transmissivity of the pellicle falls below a certain set value the pellicle is considered worn out. Again this is another way of determining wear out by transmission loss.
Pellicles can be degraded by the exposure light during use. For 365 and 248 nm, this has not been a major problem, because the pellicle materials in use are very resistant to damage at those wavelengths. At increasingly short wavelengths, such as 193 and 157 nm, pellicles are much more easily damaged. Damage can show up as a change in transmission, thickness, index of refraction, or a combination of all three. Changes in pellicle transmission lead to dose changes during wafer exposure. This will cause image size changes which can readily be detected.